1. Field of the Invention
This invention relates to photographic cameras having electromagnetic operation control and, more particularly, to an improvement in the exposure control accuracy of the electromagnetic control device by making uniform the time interval during which the actuating process is performed.
2. Description of the Prior Art
Conventional photographic cameras are generally equipped with diaphragm mechanisms either of the type where the task of introducing a proper diaphragm aperture value for the photographing lens is completed by manual setting of the diaphragm control ring provided in the lens mount barrel, or of the type where the position of the deflected pointer of the measuring instrument of the exposure meter system is scanned and the scanning result is introduced into the diaphragm mechanism through a mechanical control device. Recently, however, a new type of camera has been proposed in which the exposure value derived from an exposure control circuit is introduced into the diaphragm mechanism through a canning mechanism of which the operation is electromagnetically terminated by an arresting device as the diaphragm mechanism is automatically adjusted in position from the fully open aperture to a proper aperture value dependent upon the computed exposure value. With such an electromagnetically operated diaphragm control mechanism, it is required that when the solenoid of the electromagnet is energized, the armature must be retained by the yoke at an accurately controlled location, or otherwise the arrested position of the scanning mechanism would be varied with variation of the resultant diaphragm aperture value despite the fact that the exposure value is maintained equivalent, whereby the accuracy of exposure control is impaired. This requirement, however, can not be fulfilled in a prior art electromagnetically operated arresting device shown in FIG. 1, mainly because the arresting lever 2 is directly biased by a strong driving spring 2a to urge the armature 1b for movement away from the yoke 1a of the electromagnet 1. The arresting lever 2 has a rectangularly bent portion at the opposite end thereof to that carrying the armature 1b and is arranged to arrest at the bent portion an ever-rotating stop wheel 3 constituting part of the scanning mechanism when the solenoid 1c is deenergized after a cocking and release member 4 is released from the cocked, or latched position to turn counter-clockwise. The scanning result is introduced to the lens aperture mechanism shown in FIG. 3 through a control lever 231-and-diaphragm presetting ring 202 connection. The arrested position of the scanning mechanism is dependent upon the exposure value derived from an exposure control circuit of FIG. 4. After an exposure with the thus-adjusted diaphragm mechanism has been completed, a film winding lever may be cocked to turn a lever 5 of FIG. 1. Such movement of lever 5 causes clockwise movement of the lever 4 which in turn causes clockwise movement of the arresting lever 2. When the lever 4 is latched by a hook-shaped end 6a of a lever 6, the armature 1b is positioned closest to the yoke 1a but not in intimate contact with the yoke 1a. This gap constitutes a source for reducing the accuracy of automatic diaphragm control because of the following reasons. When the camera is released, the solenoid 1c is energized to attract the armature 1b and at the same time the scanning mechanism is actuated to initiate rotation of the stop wheel 3. At the termination of duration of a time interval dependent upon the exposure value derived from the exposure control circuit, the solenoid 1c is deenergized causing the arresting lever 2 to be turned counter-clockwise under the driving effect of the spring 2a until the bent portion of lever 2 engages one tooth of the stop wheel 3, whereby the automatic formation of the proper diaphragm aperture is effected. The duration of the actuating or a scanning time period is, therefore, largely affected by the gap between the armature and the yoke in the cocked position. This gap varies in magnitude from camera to camera depending upon the degree of accidental errors from the tolerance limits within which the various moving parts of the arresting device are to be manufactured and assembled, as the arresting lever 2 is biased by the driving spring 2a in the opposite direction to that in which the armature 1b can be brought into intimate contact with the yoke 1a. Another source for assisting in the gap variation is the different magnitude of force under which the film winding or cocking lever is operated to reset the arresting device.
Such prior art arresting device has another drawback that, as the driving spring of strong force is directly connected to the arresting lever, it is required for the electromagnet to provide a correspondingly strong force of attraction for the armature, or otherwise a mechanical shock which occurs when the camera is released would cause movement of the armature away from the yoke despite the energization of the solenoid, resulting in the overexposure of the photographic film. As the attracting force increases, the actuating or scanning time period is unavoidably increased with increase in the probability of letting a good shutter opportunity slip.